How you say that JVM does not guarantee

I read that You can't force GC, but could request it by calling System.gc().
JVM does not guarantee that GC will be started immediately.
My question is how you say JVM doesnot guarantee, can we check that.

What do you mean exactly? Are you asking what the sentence "does not guarantee" means?

It just means exactly what it says: if you call System.gc(), the JVM might or might not perform a garbage collection immediately. According to the Java specification it is not required to do a garbage collection if you call System.gc(). It might, but you can't be sure.

Note that normally you should not call System.gc(). It's only for special circumstances, when you know there is a problem (for example, you've noticed that the program pauses at a certain moment because the JVM is doing garbage collection at an inconvenient moment). In that case calling System.gc() at an appropriate moment might help. But almost always you should just leave the garbage collector alone, because in general it does its job well without needing intervention. (In fact, you might even make garbage collection less efficient by calling System.gc()).